1. Technical Field
The present disclosure relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Recently, a demand for an increase in capacity of a nonaqueous electrolyte secondary battery has been increasing. The nonaqueous electrolyte secondary battery is generally produced by winding or laminating sheet-like positive electrode and negative electrode with a separator therebetween, of a fine porous film made of, for example, polyolefin to form an electrode assembly, putting the formed electrode assembly in an outer package, pouring a nonaqueous electrolyte into the outer package, and then sealing the package. Battery capacity can be increased through an increase in density of electrode materials, a reduction in thickness of, for example, the outer package or the separator, and an increase in charging voltage of the battery voltage. Among these methods, the increase in charging voltage is a useful technique because it can increase the capacity without changing the structure of the battery. However, an increase in charging voltage may cause dissolution or deterioration of the materials constituting the nonaqueous electrolyte secondary battery. Accordingly, it is difficult to increase the charging voltage to be higher than a predetermined value.
For example, in a lithium ion secondary battery including a positive electrode of lithium cobaltate, a negative electrode of a carbon material, and a separator of polyethylene as an embodiment of a nonaqueous electrolyte secondary battery, the charging voltage is set to 4.2 V or less. One reason of this is that a battery voltage of higher than 4.2 V at the completion of the charging oxidizes the separator made of polyethylene to degrade the separator and has a risk of generating a gas by the oxidization (Japanese Unexamined Patent Application Publication No. 2001-273880). Recently, positive electrode active materials that can be charged at a voltage higher than that in lithium cobaltate have been developed, and the use of such positive electrode active materials can increase the capacity. The capacity, however, cannot be increased due to the material of the separator. In order to solve this problem, Japanese Unexamined Patent Application Publication No. 2008-210573 proposes an inorganic oxide particle layer disposed on the surface of the separator facing the positive electrode.